The present invention generally relates to packaging and more particularly to an apparatus and method for making unitary packages which hold a plurality of components, each package containing a load wrapped in a web of stretched film.
Case packing or boxing is a common way of shipping multiple unit products. The multiple unit products are generally stacked in a corrugated box or are wrapped with kraft paper with the ends of the kraft paper being glued or taped. Another way of shipping such products is by putting a sleeve or covering of heat shrinkable film around the products and shrinking the sleeve to form a unitized package. The use of heat shrinkable film is described in U.S. Pat. Nos. 3,793,798; 3,626,645; 3,590,509 and 3,514,920. A discussion of this art is set forth in U.S. Pat. No. 3,867,806.
Another common method of wrapping loads is with rotary stretch wrapping machines. These rotary machines are commonly referred to as spiral or full-web machines, and can operate with the load rotating pulling stretched film web around it. Alternately, the load can be stationary and stretched film wrapped around the load with a rotating film dispenser.
The use of spiral wrapping machinery is well known in the art and such apparatus is typified by U.S. Pat. Nos. 3,003,297; 3,788,199; 3,863,425 and 4,136,501.
Additional references of interest which are pertinent to rotatable drives for wrapping packages are disclosed in U.S. Pat. Nos. 3,820,451; 3,331,312; 3,324,789; 3,309,839; 3,207,060; 2,743,562; 2,630,751; 2,330,629; 2,054,603 and 2,124,770.
A typical state-of-the-art full web apparatus is disclosed in U.S. Pat. No. 3,867,806. A similar full web apparatus using a tensioned cling film wrapped around a rotating load is shown in U.S. Pat. No. 3,986,611 while another apparatus using a tacky PVC film is disclosed in U.S. Pat. No. 3,795,086.
Stationary loads which are brought to a loading area and are wrapped by a rotating member which dispenses stretched film around a load are disclosed in U.S. Pat. Nos. 4,079,565 and 4,109,445. U.S. Pat. No. 4,079,565 discloses a full web vertical wrap of the load, while U.S. Pat. No. 4,109,445 discloses the horizontal spiral wrap of the load.
The elasticity of the stretched plastic film holds the products of the load under more tension than either the shrink wrap or kraft wrap, particularly with products which settle when packaged. The effectiveness of stretched plastic film in holding a load together is a function of the containment or stretch force being placed on the load and the ultimate strength of the total layered film wrap. These two functions are determined by the modulus or hardness of the film after stretch has occurred and the ultimate strength of the film after application. Containment force is currently achieved by maximizing elongation until just below a critical point where breaking of the film occurs. Virtually all stretch films on the market today including products of Mobil Chemical Company (Mobil-X, Mobil-C and Mobil-H), Borden Resinite Division PS-26, Consolidated Thermoplastics, Presto, PPD and others are consistently stretched less than thirty percent in applications because of irregularities in film braking systems.
The film stretching means on all currently marketed pallet stretch wrapping devices employ either direct or indirect friction to restrict the film as it is being wound onto the load during the wrapping process. The restriction is either applied to the roll of film itself (direct friction) or applied to the film after it is unwound from the film roll (indirect friction). The pallet and load serve as the winding mandrel providing all of the pulling force required to elongate the film.
The earliest type of stretch wrapper utilized a direct friction device in the form of a brake that is connected to the film roll through the core. The torque from the frictional brake device acts on the center of the film roll and as the roll changes diameter, the voltage to the brake is altered, either by the operator or automatically by a sensing device. A later film roll brake device illustrated by U.S. Pat. No. 4,077,179, utilizes a frictional brake attached to a shaft with a roller which is pressed against the freely mounted film roll. The film roll brake eliminates the need to change the brake force during the consumption of the film roll.
Various prior art indirect friction film stretching devices have been employed to restrict the film as it is wound onto the pallet during the wrapping process. One of these devices, commonly referred to as an "S" type roller device, utilizes an idle roller followed by a braked roller over which the film is threaded prior to wrapping the load. The function of the two rollers is to align the film for maximum contact with the braked roller. Another indirect friction device having fixed bars was marketed by Radient Engineering Corporation under the trade name POS-A-TENSIONER and has been subsequently marketed by the Kaufman Company under the trade name TNT. This device has a series of fixed non-rotating bars positioned adjacent to the film roll. The film web is threaded around the bars whose relative angles can be changes for ultimate tensioning. As the film web is attached to the pallet it is drawn across the bars and the friction between the film and the smooth surface of the bars provides a restriction causing the film to stretch. This device uses multiple bars with the film web stretching incrementally between each bar. Neck down of the film web increases between each bar and the load bears the force. As the load rotates, the wrap angle changes from the last bar so that the wrapping force greatly varies depending on the relative angles. The frictional restraint is determined by the vector of the film web on each bar. Thus, the device is very sensitive to the force placed on the unwind roll and the force increases as the roll size decreases adding additional force on the system. Furthermore, there must be some friction placed on the supply roll to prevent backlash. While this device solves to some degree the irregularities of the brake and the hostility of the film roll, it can only apply limited stretch to the load and does not handle different film compositions with any degree of standardization.
Another stretch wrapper device was introduced by the Anderson Company at the PMMI Show in Chicago in 1978. This device interconnected the turntable drive motor with a pair of nip rollers immediately downstream from the film unwind roll. The nip rollers were synchronously driven with the turntable rotation through a variable transmission which could be increased or decreased in speed relative to the turntable rotation speed. Thus the stretch on the film was affected between the constant-speed nip rollers and the pallet turning. It is not known if this machine was ever commercialized, principally because of its inability to achieve satisfactory stretch over the load corners due to its failure to respond to the speed change that these corners represented. The pallet, as the film accumulating mandrel, provided the total force that was required to stretch the film from the driven nip rollers with all of the stretch occuring after the passage of the single pair of nip rollers to the pallet.
In addition to the previously noted prior art, direct friction pallet stretch wrapping machines of the pass through type have been manufactured by Weldotron and Arenco (Model No. MIPAC). These machines have a significant problem in stretching the film and normally stretch film around the load in the range of about five to ten percent by driving the pallet and associated load through a stretched curtain of film to place the stretching force on the front or sides of the load.
A typical pass through system is disclosed in U.S. Pat. No. 3,596,434. In this reference, a pallet load is transported along a conveyor and the leading face of the pallet load contacts a vertical curtain of film web formed by heat sealing leading ends of film webs dispensed by two rolls of film on opposite sides of the path of the pallet load. The pallet load continues to move along the conveyor, carrying with it the sealed film curtain until the two side faces of the pallet load as well as the front face are covered by film web. A pair of clamping jaws then close behind the pallet load, bringing the two film web portions trailing from the side faces of the pallet load into contact with one another behind the pallet. The jaws then seal the film web portions together along two vertical lines, and cut the film web portions between those two seals. Thus, the film web portions are connected to cover the trailing face of the pallet load, and the curtain across the conveyor is re-established to receive the next pallet load. The pallet load may subsequently be exposed to heat in order to shrink the film web and apply unitizing tension to the load, as is disclosed in U.S. Pat. No. 3,662,512. Another disclosure of relevance to pass through wrapping is U.S. Pat. No. 3,640,048 which shows that film may be applied to the top and bottom of the pallet load prior to the wrapping cycle when it is desired to cover all six surfaces of the pallet load with film.
Since most pallet loads will not hold together while being subjected to these unequal forces, the film web is normally tensioned after the film jaws begin their inward travel over the end of the pallet load. This form of tensioning severely limits the degree of elongation of film which is able to be achieved, and pulls excess film around the two rear corners of the load while the jaws are closing. This frequently causes film tears when the film is stretched more than ten percent.
An inherent problem with pass through packaging is that the primary strength and reliability of the package is determined by the consistent quality of the seal. These seals depend on a careful maintenance of the sealing jaw and are never as strong as the film itself. As previously mentioned, stretched film webs are difficult to seal because the stretched film is oriented and the seals can be torn by the stretch forces on the film or load shifting within the package wrap. In the prior art pass through machines, the stretch on the wrap which may be as much as ten percent, takes place at the seal with very little stretch being imparted around the load. In the present invention, there may be one hundred percent elongation around the load and zero percent elongation at the seal, resulting in unoriented film which will seal easier.
When low stretch rates of one to ten percent are produced, several packaging problems occur. The unitizing containment forces on the load are less than the optimum force which can be obtained. This minimizing of containment forces can result in a potential loosening of the film wrap during shipment where the load settles and moves together thereby reducing the girth.
Another problem is that non-vertical sides and corners on an irregular load places extreme force on a small area of film during stretching, thereby causing a partial rupture at a point well below the force achieveable on a flat side. This partial rupture causes a transfer of force to the remaining portion of the web which is frequently sufficient to produce a "zippering" of the entire film web.
One attempt to overcome these problems is seen in French Pat. No. 2,281,275 assigned to SAT. The '275 reference discloses the prestretching of plastic film by taking the film web from the film roll through a powered roller system having a speed differential of V.sub.2 -V.sub.1 which stretches the film. The film leaving the second set of rollers is drawn off at a speed which is equal to or less than V.sub.2 as it is wrapped around the load. V, which is the speed of rotation of the pallet load, is less than or equal to V.sub.2, the speed of the stretched film coming off of the second roller assembly.
Although the French reference appears to achieve film web stretch in excess of the one to ten percent range obtained in the aforementioned pass through stretch wrapping machines, other problems remain. The system requires manual operation or complex automatic feedback to accomodate the change in film take-up speed as the pallet load surfaces pass by the downstream rollers. This reference does not teach the benefit of stretching the film above the yield point with increased strength per cross-sectional area and increase in modulus. There is furthermore no teaching of reducing the force on the portion of the film web between the downstream powered rollers and the load with inelastic strain recovery as a technique for reducing wrapping force while holding high levels of elongation.
A commercial model based on FIG. 8 of the '275 reference is currently being marketed by SAT. In this embodiment the film web is pre-stretched by extending a pair of rollers forward while braking the film rolls. The load is carried into the pre-stretched "U" shaped sleeve and the rollers are transported back of the load allowing the sleeve to engage the load. Sealer bars are then projected inward to seal the web ends together.
The present invention is constructed to overcome these difficulties which are present in the prior art devices.